Esters of phthalic acid are used to a great extent as plasticizers, in particular for polyvinyl chloride. The principal alcohol components are primary alcohols having 8 to 10 carbon atoms; the most important of these is currently 2-ethylhexanol. Although phthalic esters of short-chain alcohols lead to plasticizers having good gelling capacity, their higher volatility is disadvantageous. Longer chain esters, on the other hand, gel more slowly and have a poorer cold resistance.
The properties of the phthalic ester plasticizers are influenced, not only by the size of the alcohol molecule, but also by the branching of the carbon chain. Thus, alcohols having little branching give ester plasticizers having high cold flexibility; substantially linear alcohols having 8 to 10 carbon atoms in the molecule are thus increasing in importance as the alcohol component. A precondition for their use is that they are available in large amounts and at favorable cost.
According to German Patent 28 55 421, phthalates of nine-carbon alcohols are used as plasticizers and are obtained by oxo reaction of eight carbon olefins, hydrogenation of the reaction product, and esterification of the resultant nine-carbon alcohols with phthalic anhydride. 3% to 20% by weight of the starting olefins have an isobutane skeleton in each molecule chain, less than 3% by weight of the olefins have quaternary carbon, and more than 90% by weight of the total amount of the olefins are present as n-octenes, monomethylheptenes, and dimethylhexenes. In addition, the weight ratio of the total amount of the n-octenes and monomethylheptenes to the dimethylhexenes is more than 0.8.
Phthalic esters based on ten-carbon alcohols are an object of European Patent Application 366,089. These alcohols are used in the form of a mixture which is obtained by hydroformylation of a butene fraction, aldol condensation of the resulting aldehyde mixture, and subsequent hydrogenation.
Another route to obtain a didecyl phthalate mixture is described in European Patent Application 424,767. The preparation of the esters is carried out in a multi-stage process by dimerization of butene mixtures, hydroformylation and hydrogenation of the resulting octene mixture to give a nonanol mixture dehydration of the nonanol mixture to a nonene mixture, and hydroformylation and hydrogenation of the nonene mixture, thereby to form a decanol mixture.
The known processes do not fulfill all the economic and technical requirements of a process carried out on an industrial scale, because the starting materials are not available in sufficient quantity and/or are not available at favorable costs, and/or the conversion of the starting materials into the alcohols is associated with processes which are too expensive.